Infrared cameras can acquire images even in circumstances which appear completely dark to the human eye. Such infrared cameras have application in many fields including stationary and mobile night-vision systems, covert surveillance, and the like. A complete night-vision system includes an infrared camera and a source of infrared illumination. Various types of infrared illumination sources have been proposed.
Some infrared illumination sources generate infrared light using an incandescent bulb. As the incandescent bulb emits light having a broad range of wavelengths, a filter may be provided to filter visible light from the output. Such illumination sources have the disadvantages that they require large amounts of electrical power and are relatively inefficient.
Laser diodes which emit light at infrared wavelengths are now available. Such laser diodes are relatively efficient at converting electrical power into infrared illumination but are undesirably expensive for many applications.
Light emitting diodes (LEDs) which emit infrared radiation are also available. Such light emitting diodes are not particularly bright. Therefore, their use is typically limited to illumination over shorter ranges such as a few meters. Further, the efficiency of infrared LEDs varies with temperature. The efficiency drops off at temperatures which are too high. Some proposed infrared illumination systems use arrays of infrared LEDs to create brighter illumination. In such systems temperature control becomes a problem since the infrared LEDs generate heat as well as infrared radiation.
There is a need for cost effective longer range infrared illuminators.